It's the #620. It's shorter and slimmer than the full size, has a GP steel nib but is still piston filled and a joy to write with.

Originally they came boxed with a bottle of ink and I seldom see them for sale.† They were made at least from the 70s and 80s right through the 90s. I've seen pretty used ones selling for $20-30 and NOS ones from $75-100 if with the original box and opened ink.